There are several types of treatments in which blood is extracted in an extracorporeal blood circuit. Such treatments involve, for example, haemodialysis, haemofiltration, haemodiafiltration, plasmapheresis, blood component separation, blood oxygenation, etc. Normally, blood is removed from a blood vessel at an access site and returned to the same blood vessel or at another location in the body.
In the case of haemodialysis, a treatment fluid (also referred to as a dialysis fluid) is made approximately isotonic with a patient's blood, each flowing on each side of a semi-permeable membrane of a membrane device (referred to as a dialyzer), so that, during the diffusive transfer which is established across the membrane in the case of substances having different concentrations on either side of the membrane, the impurities in the blood (urea, creatinine, etc.) migrate from the blood into the treatment fluid. The electrolyte concentration of the treatment fluid is also generally chosen so as to correct for the electrolyte concentration of the patient's blood.
In treatment by haemodiafiltration, a convective transfer by ultrafiltration, resulting from a positive pressure difference created between the blood side and the treatment fluid side of the membrane, is added to the diffusive transfer obtained by dialysis.
An apparatus for extracorporeal blood treatment includes a stage in which the disposable extracorporeal blood circuit is coupled to a treatment control monitor (for example a dialysis monitor). This stage, which is prepared before connecting up the extracorporeal blood circuit to the patient, includes connection of the blood transport lines (in general an arterial line for blood removal from the patient, and a venous line for blood return to the patient) to the membrane device for blood treatment, which in turn is connected up to the treatment fluid supply circuit and to a used treatment fluid discharge circuit.
The semi-permeable membrane of the membrane device divides a blood compartment, connected to the blood transport lines, and a fluid compartment, connected to the supply and discharge circuits. The blood transport lines are further coupled to a sensor and actuator system equipped on the treatment control monitor, which system normally comprises means for blood circulation, pressure sensors, air bubble sensor, one or more circuit blocking clamps, blood detector, etc.
Before connection of the extracorporeal blood circuit to the patient's vascular system, a priming stage is usually performed of the blood transport lines and the membrane device, which then are filled with a priming liquid (usually an isotonic saline solution or another patient-isotonic liquid). The priming stage performs the function of expelling air, filling and rinsing.